Pain(T) In the Neck – Computed Tomography Myelography of the Cervical Spine
Clinical Connections – Autumn 2022
Maty Looijen, Equine Resident
Clinical problems associated with the cervical spine in horses are common. They involve a wide variety of symptoms, including neurological deficits (e.g. ataxia, dysmetria, paresis), neck pain and stiffness, and gait abnormalities. Neurological abnormalities most commonly result from spinal cord abnormalities (e.g. infection/inflammatory conditions, neoplasia or compression).
Investigation of horses with neck problems can involve a neurological and/or lameness examination, blood tests, a cerebrospinal fluid tap or an EMG (electromyography – electronic muscle stimulant test). Radiography and ultrasonography may be helpful tools in investigating the cervical spine, however they lack sensitivity and specificity.
Radiographic examinations are commonly performed for the evaluation of potential bone abnormalities and alignment of the vertebrae. If abnormal findings are seen on radiographs, a further challenge is to distinguish their clinical significance. This is similar for ultrasonographic examinations, which are commonly performed to further investigate the soft tissues associated with the cervical spine. The neck has a complex anatomy, and radiography or ultrasonography are not able to visualise details of the deeper structures (e.g. the spinal cord). Both modalities therefore often lack the possibility to establish a final diagnosis.
Radiographic myelography of the cervical spine is a technique where an extradural contrast agent is injected into the subarachnoid space (where the cerebrospinal fluid protects the spinal cord). It is historically used to identify spinal cord compression causing ataxia. However, the myelographic contrast column abnormalities associated with the abnormality of the vertebral canal can only be accessed in a single dorsoventral orientation on radiographs.
MRI and CT are much better modalities to evaluate and further identify the cause of spinal cord compression, due to their 3D compilation of the anatomy. MRI of the cervical spine in horses is currently not available, due to the limited size of the magnet. CT, however, has become popular over the last decade. Additionally, the spinal cord can be highlighted by CT myelography, which now can be investigated in a lateromedial and craniocaudal direction as well as dorsoventral. By visualising lesions accurately, we can improve our understanding of cervical spine pathologies, treat lesions more precisely and develop new treatments.
A CT myelography of the neck at the RVC is performed under general anaesthesia to avoid side effects and improve image quality. Contrast medium is injected into the subarachnoid space, at the atlanto-occipital articulation (figure).
It diffuses in the vertebral canal caudally and thereby outlines the spinal cord. Interpretation is done by our radiologists via subjective evaluation and objective measurements, indicating compression of the spinal cord and/or the surrounding contrast column. In contrast to radiographic myelography, CT myelography can establish differences in height and width of the contrast column and spinal cord. CT myelography helps us to localise and definitively diagnose of spinal cord compression within the neck.
There are a variety reasons for compression, but cervical vertebral malformation is one of the most common causes of ataxia in sport horses. It is thought to be a developmental abnormality caused by genetic predispositions and environmental influences, such as diet, growth rate, workload and trauma. The pathophysiology of the disease involves spinal cord compression due to malformation and/or malarticulation of the vertebrae, static or dynamic instability of the vertebral canal, soft tissue or bony changes of the cervical vertebral bodies, their articulations and associated soft tissue structures.
Furthermore, we have to distinguish abnormalities which originate from either degenerative or acquired lesions, such as osteoarthritis, fractures, (sub)luxations, intervertebral disc disease or neoplasias. Osteoarthritis of the articular process joints does not necessarily implement compression on the spinal cord, this depends on their size and shape in relation the position of the spinal cord in the vertebral canal. Lastly, soft tissue abnormalities commonly found in relation to spinal cord compression involve synovial cysts, epidural haematomata or ligamentum flavum thickening – all which we now diagnose with the help of CT myelography.
Interesting future perspectives include the investigation of objective and reliable decision criteria to differentiate horses with cervical spine abnormalities and spinal cord compressions from those without.